Method of crucible-free zone melting of semiconductor material,particularly silicon



y & 1969 K. REUSCHEL 3,454,367

vME'II'IOD. OF CRUCIBLE-FREE ZONE MELTING OF SEMICONDUCTOR MATERIAL, PARTICULARLY SILICON Filed Oct. 19, 1965 United States Patent METHOD OF CRUCIBLE-FREE ZONE MELTING OF SEMICONDUCTOR MATERIAL, PARTICULARLY SILICON Konrad Reuschel, Pretzfeld, Germany, assignor to Siemens Aktiengesellschaft, Germany, a corporation of Germany Filed Oct. 19, 1965, Ser. No. 497,681

Claims priority, applicatigosn Germany, Jan. 29, 1965,

1m. 01. B01j 17/10,- C01b 33/02 US. Cl. 23301 3 Claims ABSTRACT OF THE DISCLOSURE recrystallizing from the melting zone.

My invention relates to method of crucible-free zone melting of semiconductor material, particularly silicon.

Crucible-free zone melting is employed among other processes for producing semiconductor material of high purity for electronic applications. A rod-shaped member of material to be processed, for example of silicon, is conventionally gripped at its ends and held perpendicularly in a zone melting apparatus, for example in a vacuum chamber or in a chamber filled with protective gas. A small portion of the rod length is heated by a heat source, for example by an induction heating coil or by a radiation heating source, in such a manner that a melting zone is formed at that location of the rod. By relative motion between the heating source and the rod-shaped member being processed, the melting zone is passed from one to the other end of the rod. If a seed crystal, eg a monocrystalline rod portion grown in a preceding process, is applied at one end of the semiconductor rod, the entire rod-shaped member can be transformed into a monocrystal beginning at the location of this seed crystal. By passing the melting zone repeatedly through the semiconductor rod, the concentration of impurities in the material being processed is reduced or made uniform by special measures. By various means the cross-sectional area of the member being processed can be controlled or regulated. Thus, for example by moving the end holders of the rod toward one another, the melting zone can be compressed so that the cross section of the melting zone is increased whereby the crosssection of the rod portion growing out of the melting zone is accordingly increased. Conversely, for example by moving the rod holders away from one another, the melting zone is stretched or elongated so that the cross section of the melting zone is reduced, whereby the cross section of the rod portion growing out of the melting zone is also reduced. Such a method is known, for example, from US. Patent 2,913,561. This process of compressing and stretching the melting zone can be regulated in response to various measurement values, for example in response to the current supplied to the heating coil or in response to optical measurement values. In general, the regulation is carried out in such a way that the cross section of the rod is kept uniform over the entire rod length whereby wafers sliced from the rod perpendicularly to the rod axis are able to be further processed to semiconductor components without requiring further division or trimming because all of the wafers have the same area size.

In order to process the semiconductor wafers further to ice semiconductor components, not only the purity of the material but also the crystal quality thereof is important. In most cases it is desired tht crystal dislocations should be present only in very small quantities and furthermore that they should be distributed relatively uniformly over the rod cross section. There has been recent success in producing monocrystalline material which is completely free of dislocations, however this is not desirable in every case. Thus it has been found to be advisable for example in the manufacture of components by alloying, that a small amount of dislocations be resent in the semiconductor material.

It is accordingly an object of my invention to provide a method of producing semiconductor material by crucible-free zone melting which improves the crystal quality of the semiconductor material over that produced by the methods heretofore employed.

With the foregoing and other objects in view I provide a method of crucible-free zone melting of semiconductor materials, particularly silicon, wherein a rod-shaped member of the material to be processed is supported substantially vertically at its ends and a melting zone is passed along the length of the rod from one of the ends of the rod to which a seed crystal has been fused. A characteristic feature of the invention is that the size of the cross section of the rod-shaped member is controlled at least for the last pass of the melting zone through the rod so that the cross section of the rod continually increases in direction from the seed crystal to the remote rod end.

Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as method of crucible-free zone melting. semiconductor material, particularly silicon, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The method of the invention, together with additional objects and advantages thereof will be best understood from the following description when read in connection with the accompanying single figure of a drawing showing a diagrammatic view of apparatus for carrying out the method of the invention.

Referring now to the drawing there is shown a semiconductor rod 2 which is held vertically, as shown in the figure, by its ends in holders 3 and 4. A seed crystal 5 is fused at the lower end of the semiconductor rod 2. The seed crystal 5 expediently has a smaller diameter than that of the semiconductor rod 2 which is being processed to improve the quality thereof. The upper rod holder 3, for example, can be raised and lowered and thereby the melting zone 6 can be respectively stretched and compressed. The lower rod holder 4 is suitably rotatable about its own axis so that the growing semiconductor material is forced to assume a circular cross section. The melting zone 6 is produced by a heating coil 7 which heats the semiconductor material inductively. The travel direction of the heating coil 7 and the melting zone 6 consequently is from the bottom to the top, as shown in the figure, in a direction away from the seed crystal 5.

It has been noted that for very large rod cross sections, for example for rods with diameters of more than 30 mm., difiiculties with regard to the crystal quality have occcurred. If due to specific measures, such as for example the selection of seed crystal, and due to corresponding cooling and heating characteristics, such as by the selection of the travel speed of the melting zone for example, the dislocation density i.e. the number of dislocations relative to the unit area of cross section, in the rod portion extending from the seed crystal attains the desired value for example of 50,000 dislocations per cm.

the dislocation density will increase with increasing distance along the rod from the seed crystal and will have a value at the end of the rod distant from the seed crystal which is more than twice that at the seed crystal end.

It has been determined that in the case where the cross section of the semiconductor rod increases with the dis tance from the seed crystal, the dislocation density can be maintained practically uniform and, when measured over the rod cross section, will also have no greater deviations or variations than by a factor of 1.5. As has been further determinated, it is unnecessary in most cases for the semiconductor rod to be provided with longitudinally increasing cross-sectional area during the entire zone melting process for repeated passes of the melting zone but rather, such an increasing cross-sectional form or taper can be produced solely during the last pass of the melting zone. Provision can consequently be made for a semiconductor rod produced in another manner, for example by deposition from the gas phase, to maintain a uniform cross-sectional area over its entire rod length by automatic regultaion during the greater part of zone melting process, while the size of the cross-sectional area is controlled by compression only during the last zone pass to obtain the desired taper effect. Frequently a conical or tapering monocrystal suitable for subsequent processing to semiconductor components can be produced from a polycrystalline rod with only one pass of the melting zone. There is a relationship between the degree of increase in cross section of the semiconductor rod and the attainable effect in that a greater increase in cross section is accompanied by a greater reduction in the density of dislocations. With regard to round or circular cross sections it is suflicient in most cases for the diameter to increase about 5 mm. for a rod length of about 50 mm. so as to achieve the desired objective, namely a uniform dislocation density over the entire rod length. An increase of the rod diameter should not be less than 1 mm. for a rod length of 200 mm. and an increase of 1 mm. in rod diameter for 1 mm. of rod length should not be exceeded.

It is also important that during the pass of the melting zone in which the conical form of the monocrystal is obtained, consideration should be given to the fact that the slight increase of the cross-sectional area should not be interrupted or broken even momentarily at any location for example by transverse grooves, constrictions or the like. Consequently the travelling speed of the melting zone for example should not be increased above 5 mm. per minute because, above that speed, the formation of very large transverse grooves or fissures may be anticipated, which, in such a case, will produce short negative slopes of the rod. The lower limit of the melting zone travel speed appears necessarily to be 1 mm./min.

It has been found in the aforedescribed method of my invention that attention should be given to maintaining the dislocation density uniform within desir imits, both over the rod cross section as well as over the rod length, for example at between 10,000 and 80,000/cm. The formation of damaging lineages is also prevented, and slippages appear only within relatively small permissible limits. The method of my invention is particularly suitable for processing rods of large cross section, for example of 35 or 40 mm. diameter and more, wherein greater danger exists that the dislocation density will increase with the distance from the seed crystal and that lineages will occur especially at the margin or edge of the cross-sectional area.

I claim:

1. Method of crucible-free zone melting semiconductor material to form a crystalline member having a substantially uniform dislocation density which comprises supporting a rod-shaped member of semiconductor material substantially vertically by a holder at an end thereof, fusing one end of a seed crystal, supported by a holder at the other end thereof, to the free end of the rod-shaped member, passing a melting zone at least once along the rod-shaped member by moving the rod-shaped member and a heating device forming the melting zone in the rodshaped member relative to one another, and simultaneously moving the end holders toward one another at a constantly increasing speed as the melting zone is being passed through the rod-shaped member so that the cross section of the rod-shaped member recrystallizing from the melting zone increases in the direction of movement of the melting zone through the rod-shaped member whereby dislocations normally increasing in number with increasing distance along a recrystallizing rod-shaped member of constant cross section will have a uniform density along the recrystallized rod-shaped member of increasing cross section.

2. Method according to claim 1 wherein the end holders are moved toward one another at a speed increasing the diameter of the rod-shaped member at a ratio of rod diameter to rod length ranging between 1:1 and 1:200.

3. Method according to claim 1 wherein the melting zone is passed several times through the rod-shaped member, the end holders being moved toward one another at uniform speed in the direction of movement of the melting zone during all of the passes except the final pass.

References Cited UNITED STATES PATENTS 2,743,200 4/1956 Honnay 23301 2,985,519 5/1961 Kellmen 23-273 2,992,311 7/ 1961 Keller 23-301 3,206,286 9/1965 Bennett 23-601 NORMAN YUDKOFF, Primary Examiner.

U.S. C1. X.R, 33 8 

